A traditional phased-array radar system is unsuitable for some applications due to, e.g., its size, power requirements, the number of array elements per unit area (element density), and cost. For example, the phased array of a traditional radar system is too dense and scans a field of view (FOV) too slowly, and the system is too expensive, for use in an autonomous (self-driving) automobile. Similarly, the phased array of a traditional radar system is too dense, and the system too expensive, too heavy, and too power hungry, for use in an unmanned aerial vehicle (UAV) such as a drone.
Therefore, radar systems that are lighter, are less-dense, are less-expensive, are less power hungry, and can scan a FOV more quickly, than traditional radar systems have been developed for such applications.
An example of such a radar system that has been developed for use in automobiles includes a digital beam-forming (DBF) receive-antenna array having, e.g., at least four to eight individual antenna segments (the number of antenna segments is typically limited to the number of antenna channels that the system circuitry supports, e.g., one antenna segment per antenna channel).
During a transmit period, the system circuitry effectively energizes all of the antenna segments with the same signal, i.e., with respective signals each having the same magnitude and phase, such that the array “sprays” signal energy over a fixed FOV. Put another way, the simultaneous energizing of all the antenna segments with respective signals each having the same magnitude and phase generates a main transmit beam that is stationary, i.e., not steered. In order to cover a useable FOV, this transmit main beam is often fairly wide, e.g. more than 20 degrees in azimuth (AZ).
During a receive period, the system circuitry can post-process, dynamically, a respective gain and phase shift from any receive antenna segment, so as to digitally form and steer a receive beam that is significantly narrower than the transmit beam; the system can steer the receive beam to only a single position, or to multiple positions, within a single receive period.
Unfortunately, a problem with such a radar system is that the receive DBF can be performed only within the region illuminated by the transmit beam. The number of receiver-array segments/channels that are present are utilized to divide the fixed transmit FOV into equal segments; that is, the number of receiver-array segments/channels defines the receive resolution of the DBF, and, ultimately, defines the receive resolution of the entire radar system. The receive resolution defined in this manner is often referred to as the Rayleigh resolution, and represents a fundamental limit of the radar's performance. For example, a radar system that were to illuminate an FOV of 20° in AZ on transmit and that were to include four receive channels would possess a Rayleigh resolution of about 5° across this FOV. An alternate choice could be made to widen the FOV to 40° in AZ, which, with the same four receive channels, would give a Rayleigh resolution of about 10°. Thus, a fundamental trade-off between FOV and Rayleigh resolution exists in such a system.
Designing such a radar to illuminate a large FOV in transmit and also to possess a high Rayleigh resolution in receive would require a large number of antenna segments/channels.
Unfortunately, engineering limits to the number of channels which can be practically included in such a radar has, to date, limited the Rayleigh performance of such radars systems.
One approach to improve the Rayleigh resolution of a system with a fixed number of antenna channels is to place the receive antennas/antenna segments further apart, i.e., to design a sparse receive array.
But such a sparse array can cause spatial aliasing, which produces side-lobes and grating lobes that can hinder the radar system's ability to detect, to identify, and to map objects. One reason for such aliasing-induced side-lobes and grating lobes is that the radar system's sparse receive-antenna array does not meet the Nyquist criteria for maximum segment spacing, which is λ/2. For example, to obtain a Rayleigh resolution of 1° in the AZ dimension, the antenna would need to have dimensions on the order of 50λ. Distributing a small number of segments/channels, e.g., four to eight, across a distance of 50λ would result in an average segment spacing of 6.25λ to 12.5λ, which is 12 to 25 times the maximum Nyquist spacing of λ/2. Consequently, the system would suffer from significant side-lobes and grating lobes.
Of course, to reduce spatial aliasing, a designer could reduce the effective size of the antenna by reducing the spacing between the antenna segments.
But reducing the size of the antenna limits the minimum width of the receive beam that the radar system could generate.
Therefore a designer of such a sparse-antenna-array radar system (i.e., a radar system with a larger antenna aperture and a limited number of antenna channels) is faced with trading off beam width for aliasing, and vice-versa. That is, the narrower the receive-beam width, the greater the level of aliasing, and the lower the level of aliasing, the wider the receive-beam width.